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. 2022 Jan 24;15(3):881.
doi: 10.3390/ma15030881.

An Investigation of a New Parameter Based on the Plastic Strain Gradient to Characterize Composite Constraint around the Crack Front at a Low Temperature

Lingyan Zhao  1 , Zheren Shi  2 , Zheng Wang  2 , Fuqiang Yang  1
Affiliations

An Investigation of a New Parameter Based on the Plastic Strain Gradient to Characterize Composite Constraint around the Crack Front at a Low Temperature

Lingyan Zhao et al. Materials (Basel). .

Abstract

Stress corrosion cracking (SCC) is an important destruction form of materials such as stainless steel, nickel-based alloy and their welded components in nuclear reactor pressure vessels and pipes. The existing popular quantitative prediction models of SCC crack growth rate are mainly influenced by fracture toughness values KJc or Jc. In particular, the composite constraint, containing the in-plane constraints and out-of-plane constraints around the crack front, has a significant influence on the fracture toughness of structures in nuclear power plants. Since the plastic strain gradient is a characterization parameter of the quantitative prediction model for crack growth rate, it may be a characterization parameter of composite constraint. On the basis of the experimental data at a low temperature of alloy steel 22NiMoCr3-7 used in nuclear pressure vessels, the gradient of equivalent plastic strain DPEEQ around the crack fronts at different constraint levels was calculated using the finite element method, which introduces a new non-dimensional constraint parameter Dp, to uniformly characterize the in-plane and out-of-plane constraint effects. Compared with constraint parameters APEEQ or Ap, the process of obtaining parameters DPEEQ or Dp is much simpler and easier. In a wide range, a single correlation curve was drawn between parameter Dp and normalized fracture toughness values KJc/Kref or Jc/Jref of specimens at a low or high constraint level. Therefore, regardless of whether the constraint levels of the structures or standard specimens are low or high, constraint parameter Dp can be used to measure their fracture toughness. To build an evaluation method that has structural integrity and safety while containing the composite constraint effects, in addition to accurate theoretical interpretation, further verification experiments, numerical simulations and detailed discussions are still needed.

Keywords: composite constraint; fracture toughness; quantitative prediction; strain gradient; stress corrosion cracking.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
Geometry sizes: (a) SE(B); (b) C(T) specimens.
Figure 2
Figure 2
Mesh of global model and mesh around crack front: (a) C(T)50 specimen and (b) SE(B)10 × 10d specimen.
Figure 3
Figure 3
Equivalent plastic strain around crack fronts in sections.
Figure 4
Figure 4
Equivalent plastic strain around crack fronts: (a) Jc = 29.65 kJ/m2, (b) Jc = 47.46 kJ/m2, (c) Jc = 61.35 kJ/m2, (d) Jc = 75.43 kJ/m2, and (e) Jc = 84.14 kJ/m2.
Figure 4
Figure 4
Equivalent plastic strain around crack fronts: (a) Jc = 29.65 kJ/m2, (b) Jc = 47.46 kJ/m2, (c) Jc = 61.35 kJ/m2, (d) Jc = 75.43 kJ/m2, and (e) Jc = 84.14 kJ/m2.
Figure 5
Figure 5
Calculated J integrals around crack fronts.
Figure 6
Figure 6
Equivalent plastic strain gradients around crack fronts.
Figure 7
Figure 7
Relation between fracture toughness KJc and constraint parameter DPEEQ.
Figure 8
Figure 8
Relation between fracture toughness Jc and constraint parameter DPEEQ.
Figure 9
Figure 9
Relation between non-dimensional fracture toughness KJc/Kref and constraint parameter Dp.
Figure 10
Figure 10
Relation between non-dimensional fracture toughness Jc/Jref and constraint parameter Dp.
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